Controlling apparatus



Dec. 14, 1965 w. D. REED 3,222,995

CONTROLLING APPARATUS Filed April 14, 1964 4 Sheets-Sheet 1 FIG. I

FINAL CONTROL ELEMENT INVENTOR. WAYN E D. REED ATTOR NEY.

Dec. 14, 1965 w. D. REED 3,222,995

CONTROLLING APPARATUS Filed April 14, 1964 4 Sheets-Sheet 2 F l G. 2 I64 FINAL CONTROL ELEMENT IN VEN'TOR.

WAYNE D. REED BY /V ATTORNEY.

Dec. 14, 1965 w. D. REED 3,222,995

CONTROLLING APPARATUS Filed April 14, 1964 4 Sheets-Sheet 5 F I G. 3 /54 /6 INVENTOR WAYNE D. REED W/VL/ZW ATTORNEY.

Dec. 14, 1965 w, REED 3,222,995

CONTROLLING APPARATUS Filed April 14, 1964 4 Sheets-Sheet 4 /0 FIG. 4

INVENTOR.

WAYNE D. REED QWh ATTOR N EY.

FINAL CONTROL ELEMENT 3,222,995 CGNTROLLING APPARATUS Wayne D. Reed, Philadelphia, Pa., assignor to Honeywell Inc., a corporation of Delaware Filed Apr. 14, 1964, er. No. 359,734 11 Claims. (Cl. 91-359) A general object of the present invention is to provide a fluid pressure actuated, single-stage, double-acting positioner.

More particularly, it is an object of the invention to provide a positioner of the aforementioned type that has two interchange-able pneumatic relay chambers, each of which has a fluid supply and an exhaust valve therein, a tie rod opera-bly connected to the supply and exhaust valves to provide a rapid increase in the pressure of the supply fluid in one chamtber while the pressure of the fluid in the other chamber is rapidly exhausted to atmospheric pressure.

It is another object of the present invention to provide a unique multi-valve relay construction of the aforementioned type that is more economical to operate than other types of double-acting positioners because of the abnormally lower level of supply air under pressure that it consumes.

It is another object of the present invention to disclose a single-stage, double-acting positioner which, because it has fewer parts than those present in other available positioners, can reduce the undesired number of non-linear errors due to friction which are introduced by the additional, unnecessary parts.

It is another object of the invention to disclose a unique, single-stage, double-acting positioner of the aforementioned type that is provided with a resilient feedback connection which provides rapid positioning of the member that the positioner is assigned to move.

It is still another object of the invention to disclose a unique, single-sta-ge, double-acting positioner of the afore mentioned type which can be rapidly converted from one which provides direct action into a positioner that will provide reverse act-ion by merely flipping over a cam one hundred eighty degrees which forms a part of a negative feedback linkage and interchanging the ends of the fluid pressure connection from the respective relay chambers that are connected to the opposite ends of a fluid-actuated piston.

A better understanding of the present invention may be had from the following detailed description when read in connection with the accompanying drawings in which:

FIG. 1 shows the position that the aforementioned cam and outlet connections will be in when the positioner is being used as a direct-acting unit and, also, shows in schematic form the aforementioned interchangeable relays;

FIG. 2 shows the position that the aforementioned cam and outlet connections will be in when the positioner is being used as a reverse-acting unit and, also, shows in schematic form the aforementioned interchangeable relays;

FIG. 3 shows in detailed form the various parts of the interchangeable relays of the positioner shown in FIGS. 1 and 2, and

FIG. 4 shows how a negative feedback, lever-actuated spring that is of a different construction than the cam and lever connection shown in FIG. 1 can be employed as the negative feedback linkage in a direct acting positioner.

The aforementioned single-stage, double-acting positioner is identified in FIGS. 1 and 2 of the drawing as reference numeral 10. This positioner is comprised of two interchangeable pneumatic relays 12, 14 rotated one hundred eighty degrees with respect to one another, a

3,222,985 Patented Dec. 14, 1965 ice controlling variable unit 16 that is associated for operation with these relays, a piston-type positioning unit 18 and a negative feedback unit 20.

The relay 12 is comprised of a rigid casing 22 that, in turn, has a non-restricted port 24 passing therethrough. The port 24 is connected in series by way of passageways 26, 28 in the rigid cylindrical parts 22 and 30 to the atmospheric exhaust passageway 32. The relay 14 is also comprised of a rigid casing 32 that, in turn, has a non-restricted port 34 passing therethrough. The port 34 is connected in series by way of passageways 36, 28 in the rigid cylindrical parts 33, 30 to the atmospheric exhaust passageway 32.

The rigid casing 22 forming relay 12 is also comprised of a fluid pressure supply port 38. This port 38 is connected in series by way of passageways 40, 42 to a fluid supply conduit 44 which, in turn, is connected to a fluid pressure supply source, not shown, that delivers, e.g. a regulated one hundred fifty pounds per square inch, filtered air supply (F.A.S.) into the left end of the passageway 44.

The rigid casing 33 forming relay 14 is also comprised of a fluid pressure supply port 46. This port 46 is also connected in series by way of the passageways 48, 50 to the previously-referred-to fluid supply conduit 44. The aforementioned open ends of ports 24, 34, 38, 46 each have a flapper 52, 54, 56 or 58 associated therewith. Each of these flappers 52, 54, 56, 58, in turn, is provided with a biasing means in the form of a leaf spring 60, 62, 64 or 66. Each of these leaf springs 60-66 is retained at its fixed end by suitable associated screw connections 68, 70, 72, and 74 to the respective parts 22, 32 of the relays 12 and 14 as shown in FIG. 3.

A common actuating member in the form of a connecting rod is shown passing through the central portion of the positioner and connected in fluid-tight engagement to the spaced-apart flexible diaphragms 78, 80, 82, 84 and 86 by means of identical pairs of diaphragm plate members 88, 90, 92 and 96.

The connecting rod 76 contains spaced-apart embossed portions 98, 100, 102, 104 which when moved to the force balance position, which is shown in FIG. 3, will permit the flappers 56, 58 and 54 with which they are engaged to allow practically no fluid to be fed into or exhausted from the chambers 106, 108 through the ports 24, 38, 34, or 46.

FIG. 3 of the drawing shows the construction of the connecting rod 76 in more detailed form than that shown in FIGS. 1 and 2. In FIG. 3 it will be noted, for example, that the rod'76 is made of three parts 110, 112, 114. The part 110 of this rod 76 is shown having an adjustable screw-threaded connection 116 at its upper end for adjustably connecting it to the diaphragm plate member 90. The other end of the connecting rod part 110 has a screw thread for accommodating the mounting of a nut 118 thereon. This nut 118 is shown retaining a cylindricallyshaped member 120, 122 and the free end of a spidershaped spring plate 124 in tight engagement thereon. The other end of this spring plate 124 and two other similar parts, not shown, that make up this spider spring are fixedly attached by a suitable welding material such as that shown at 126.

The part 112 of the connecting rod 176 is fixed by an adjustably threaded connection at 128 to the base of the cylindrical cup-shaped member 130. A seat screw 132 is employed at 134 to retain the member in a preselected fixed adjusted position on the rod 76. A lower, outer surface of the member is shown press-fitted into fixed engagement with the upper, inner wall surface of the member at the surface 135.

A disc 134 having a series of slots, for example 136, 138, in its outer periphery is shown fixedly mounted on the lower end portion of the connecting rod part 112.

3 FIG. 3 shows a surface 137 forming a slot in the wall 30 for insertion of a screw driver to engage the slotted portion 138 to adjust the connecting rod part 112. The upper end of the connecting rod part 114 is shown in FIG. 3 threadedly connected at 140 to the lower part of the diaphragm plate 94.

The lower end of the connecting rod 114 has a screw thread 142 for accommodating the mounting of a nut 144 thereon. This nut 144 is shown retaining a cylindrically shaped member 146, collar 148 and the free end of a spider-shaped spring plate 150 and two other similar parts, not shown, which make up this spider spring and which are fixedly attached by a suitable welding material such as that shown at 152.

Beside the two already-mentioned relay chambers 106, 108, in FIGS. 1 and 2 there is shown a third chamber 154. This chamber 154 has an inlet passageway passing through its stationary wall 156 and a passageway 158 extending therefrom through which a fluid pressure representative of the magnitude of a condition can be applied thereto in the direction of the arrow. The controlling or instrument fluid pressure which is applied in the aforementioned manner is varied over any suitable selected range, such as zero to thirty pounds per square inch.

It can thus be seen that an increase in the pressure of the fluid being applied to the chamber 154 will cause the diaphragm 88 and the connecting rod 76 fixedly connected thereto to move in a downward direction, whereas a decrease in this applied pressure will cause the diaphragm 88 and the connecting rod 76 to be moved in the reverse direction for the reasons hereinafter described.

It should be noted that atmospheric pressure is applied to the chambers 160, 162 by way of their associated passageways 164, 166.

The direct-acting positioner in FIG. 1 shows a passageway formed by an aperture 168 and a flexible tube 170 that is connected at 172 to apply changes occurring in the zero to one hundred fifty pounds per square inch fluid pressure in the chamber 106 to the right side of the piston 174 in the cylindircal unit 18.

The positioner shown in FIG. 1 is also provided with a passageway formed by an aperture 176 and a flexible tube 178 that is connected at 179 to apply changes occurring in the Zero to one hundred fifty pounds per square inch fluid pressure in the chamber 108 to the left side of the piston 174 of the cylindrical unit 18.

The piston 174 is provided with an O-ring 180 at its outer periphery and is mounted for slidable movement along the inner surface of the cylindrical unit 18.

A piston rod 182 is fixedly attached for movement with the piston 174 through a suitable fluid-tight seal 183. (The right end of the piston rod 182 is provided with a pm 183 that retains a serrated link 184 in a selected angular position thereon. A serrated part 185 of a channel 186 is attached to the associated serrated part of link 184 by means of a screw member 187. A pin 188 is slidably engaged along a wall surface 189 forming a wall of the channel 186. This pin 188 has a locking nut 190 to rigidly retain it in a selected position along a slotted-out wall portion of link 191. The link 191, in turn, is fixedly connected for rotation with a shaft 192 that, in turn, is supported for rotation on a stationary casing member 194. The unit 16, casing parts 22, 30, 33, 194 are retained together as an integral unit by suitable connecting means such as bolts. A link 196 is fixed at one end to the shaft 192 and is provided at its other end with threads, not shown, for supporting a cam 198 thereon by means of a screw connecting means 200, 202. A roller 204 is shown in FIG. 1 in surface-to-surface rotatable engagement with the upper surface of the cam 198. The roller 204 is mounted on a shaft 206 which, in turn, is supported in the roller support bracket 208.

The upper end of the bracket 208 is fixedly mounted on the plate 210 which is in physical compressed contact with the lower end of the res lient spring member 212.

The upper end of the spring member 212 is shown in compressed physical surface-to-surface engagement with the flange plate 196. The parts 182-212 constitute all of the negative feedback parts 20, shown in FIGS. 1 and 2.

The right end of the piston rod 182 is connected in any suitable manner to move a final control element 197 therewith.

The parts in FIG. 2 are similar to those in FIG. 1, except that the cam 198 has been turned over one hundred eighty degrees from top to bottom, and the end connections 172 and 179 have been reversed from that shown in FIG. 1 to enable the positioner to be rapidly changed from a direct-acting type to a reverse-acting type.

FIG. 1 illustrates the position the flappers 52-58 will be in when the connecting rod 76 is moved downward by an increase in the fluid pressure acting on the diaphragm 78 of the chamber 154. FIG. 2 illustrates the position that these flappers 52 58 will be in when the connecting rod 76 is moved upward from the FIG. 1 position due to a decrease in the fluid pressure acting on the diaphragm of chamber 154.

The air supply ports 38, 46 are each provided with associated flappers 56, 58 that have one end pivoted on the outer edge of their associated ports. The other end of each of these flappers 56, 58 shown in FIG. 3 is adjustably fixed in position by the range-adjusting screw means 138 for selected pivotal movement by the embossed portions 100, 102 that form parts of the connecting rod 76.

It can be seen in FIG. 1 that an increase in the fluid pressure in chamber 154 allows the full force of the spring 64 to be applied to the port end of the flapper 56 so as to move this end into a substantially one hundred per-cent fluid-tight position with the open inner end of port 38. While this action occurs, the embossed portion 102 of rod 76 will move the flapper 58 to the open position shown in FIG. 1. While this latter action occurs, the flapper 52 covering the exhaust port 24 will be moved in a similar pivoted manner to that described supra to the open position, while the spring 62 is allowed to retain the flapper 54in a substantially one hundred percent fluidtight position with the open inner end of the port 34.

It can be seen in FIG. 2 that, if the fluid pressure is decreased in the chamber 154, shown in either FIG. 1 or 2, the movement of the flappers associated with the supply ports 38, 46 will be the reverse of that just described.

It can also be seen that the position of the flapper 52 covering the open inner end of the exhaust port 24 of chamber 106 and the position of the flapper 54 covering the open inner end of the exhaust port 34 of chamber 108 will be altered from the position shown for these flappers in FIG. 1 to the position shown in FIG. 2 when the fluid pressure in chamber 154 is decreased.

From the aforementioned description of the flapper movement, it can be seen that upon an increase in pressure in chamber 154 in the direct-acting positioner of FIG. 1, the level of the pressure in passageway 178 and the left side of the piston chamber 18 will be rapidly increased while the level of the pressure in the chamber 106 will be rapidly decreased. This action will cause the link 184, screw 187 and channel 186 on the piston rod 182 to move the pin 188 and links 191, 196 counterclockwise about their pivot pin 192. This action, in turn, will cause the cam 198 to force the roller 204 in an upward direction while motion will be transmitted as a negative feedback force by spring 212 to the end of the connecting rod 76 and thereby rapidly cause the connecting rod to be brought to a force balance position.

When a similar increase in the level of the fluid pressure takes place in chamber 154, as shown in FIG. 2, the increase in fluid pressure in chamber 108 will be applied to the right end of the piston 174. This action will, in turn, cause the link 184, screw 187, channel 186 on the piston rod 182 to move the pin 188 and links 191, 196

in a clockwise direction about pin 192 and an increase in the rapidly-balancing negative feedback force to be applied by way of cam 198, roller 204 and spring 212 to the lower end of the connecting rod 76.

The positioner is used with the negative feedback linkage 214, shown in FIG. 4, in lieu of the negative feedback linkage 20, shown in FIGS. 1 and 2, when the final control element 197 is required to be moved through a distance that is more than a few inches.

FIG. 4 shows the positioner 10 connected similarly to that shown in FIG. 2, but containing a different type of negative feedback linkage 214 than that shown in the previously-described positioner. The negative feedback linkage 214 shown in FIG. 4 is comprised of a tie rod 216 threadedly connected at 218 for movement with the piston rod 184. Retained on the right end of the rod 216 there is shown a screw connection 218 that holds the vertical portion 220 of a I-shaped bracket 222 against the right end of the shaft 216. The base portion 224 of the bracket 222 is employed to retain one end of the coil spring 226 in fixed engagement with the outer surfaces of the rod 216.

The looped end 228 of the spring 226 is connected to the curved inner end 230 of a connecting rod 232. The upper end of the rod 232 is threadedly connected to the spring-retaining plate 234 which is retained in contact with the plate 146.

One end of the coil spring 236 engages the underside of the retaining plate 234 to retain it in continuous contact with plate 146. The lower end of the spring 236 is shown in engagement with the adjustable sleeve 238. Rotation of the sleeve 238 is employed as a convenient way of adjusting the zero position of the spring 226.

The sleeve 238 is in threaded engagement at 240 with the U-shaped member 242. The member 242 is fixedly connected to the stationary casing member 194 by means of the tap bolts 244, 246.

In FIG. 4, it can be seen that the piston rod 184 will transmit a negative feedback force to the end of the connecting rod 142 by means of the rod 216, bracket 224, spring 226, rod 232, and the action of the spring 236.

From the aforementioned description, it can be seen that a fluid-pressure actuated, single-stage, double-acting positioner is disclosed which provides two interacting, interchangeable pneumatic relay chambers that consume an abnormally low level of air under pressure because they each contain supply and exhaust ports, and a negative feedback linkage that provides rapid, precise, linear positioning control when it is operated as either a direct or reverse acting unit.

What is claimed is:

1. A fluid control device, comprising two pneumatic relays, a wall portion forming a separate chamber for each relay, a pair of spaced-apart apertures formed in the wall portion of each relay to respectively form a fluid inlet and exhaust port for each of the chambers, a pair of flexible members positioned in each chamber forming opposing wall portions thereof, a connecting rod extending through and in fluid-tight engagement with each of the flexible members, a third chamber formed by a stationary wall portion and one of the flexible members, an inlet passageway formed in the stationary wall portion of said third chamber for applying a fluid pressure representative of the magnitude of a condition thereto, a separate flapper positioned to substantially cover each port, a separate spring for one end of each flapper positioned to normally retain each flapper in substantial fluid-tight engagement with an inner wall surface of the chamber forming the end of its associated port, the other end of the flapper covering the exhaust port in one of the relays being operably connected for movement with the connecting rod to open the last-mentioned port while the flapper covering the inlet port in the second of the first two mentioned relays is simultaneously opened upon the occurrence of an increase in the magnitude of the fluid in the third chamber from a preselected level, the other end of the flapper covering the inlet port in said one of the relays being operably connected for movement with the connecting rod to open the last-mentioned port while the flapper covering the outlet port in the second one of the aforementioned relays is simultaneously closed upon the occurrence of a decrease in the magnitude of the fluid in said third chamber from the preselected level, a cylinder, first and second inlets adjacent opposite ends of the cylinder, a piston slidably disposed within the cylinder, a passageway connecting the fluid under pressure in said first chamber by way of the first inlet with an inner portion of the cylinder that is at one end of the piston, another passageway connecting the fluid under pressure in the second chamber by way of the second inlet with an inner portion of the cylinder that is at the other end of the piston, the piston rod being connected for movement with the piston, and a feedback spring and linkage interconnecting the piston rod with the connecting rod to apply a negative feedback force to said connecting rod.

2. A fluid pressure-to-motion transducer, comprising two relays, a chamber formed by each relay, a pair of spaced-apart flexible wall portions forming opposite ends of each chamber, a wall spaced from and forming an inclosure about an exposed surface of one of the flexible members and adapted to receive a fluid under pressure whose magnitude is varied in accordance with the magnitude of a condition, a supply and exhaust valve positioned between each pair of the flexible opposing end wall members connected to respectively introduce a flow of a supply fluid under pressure into each chamber and to exhaust the fluid therefrom, a mechanical means operably connected to each of the flexible wall members and said supply and exhaust valves for transmitting movement of said one of the flexible end wall members thereto, the exhaust valve in one of the two chambers and the supply valve in the other chamber being connected for movement toward an opened position while the other two valves remain in their closed positions upon an increase in the applied fluid pressure, the supply valve of the first one of the two chambers and the exhaust valve in the other chamber being connected for movement toward an open position while the other two valves remain in their closed position upon a decrease in the applied fluid pressure, a fourth chamber, a piston slidably movable in the fourth chamber, a first flexible connection extending between the first of said two chambers and a third chamber that is on one side of the piston, a second flexible connection extending between the other of the two chambers and a third chamber portion that is on the other side of the piston, and a flexible feedback linkage connected for movement with the piston and extending between the piston and an end of the mechanical means to apply a negative feedback force thereto.

3. A single-stage, double-acting, air-controlling positioning device, comprising two identical casings having chambers formed therein fixedly interconnected to form a unitary structure, slidable positioning means, each of the chambers having a separate passageway for applying controlled air therefrom connected to opposite sides of the slidable positioning means, an inlet passageway projecting into each chamber and forming an inlet for air thereto, an outlet passageway projecting into each chamber and forming an exhaust outlet for air therefrom, separate spring biased flappers mounted in each chamber to normally substantially seal off each of said inlet and said exhaust passageways, a common actuating element positioned to pass through said chambers 'and to move the flappers in response to a controlling variable.

4. The positioning apparatus as defined in claim 3, wherein one of the casings is rotatably displaced from the other.

5. The positioning apparatus as defined in claim 3, wherein the inlet and outlet passageways in one of the casings are rotatably displaced from the inlet and outlet passageways in the other casing.

6. The positioning apparatus as defined in claim 3, wherein one of the casings is rotatably displaced from the other and the actuating member being connected with the flappers to open the exhaust port and to retain the inlet passageway in its substantially sealed ofi position in one of the chambers while the inlet passageway is simultaneously opened and the exhaust port is retained in its substantially sealed off position in the other chamber.

7. The positioning apparatus as defined in claim 3, wherein a resilient motion-to-force transmitting connection is employed between the slidable positioning means and the actuating element to transmit a negative feedback force thereto.

8. The positioning apparatus as defined in claim 3, wherein a resilient linear motion to non-linear force transmitting connection is employed between the slidable positioning means and the actuating element to transmit a negative feedback force thereto.

9. The positioning apparatus as defined in claim 3, I

wherein the inlet and outlet passageways in one of the casings are rotatably displaced from the inlet and outlet passageways in the other casing and a resilient motion-toforce transmitting connection is employed between the slidable positioning means and the actuating element to transmit a negative feedback force thereto.

10. The positioning apparatus as defined in claim'3, wherein one of the casings in inter-connected in a reverse end-to-end relationship from the other and a resilient linear motion to non-linear force transmitting connection is employed between the slidable positioning means and the actuating element to transmit a negative feedback force thereto.

11. The positioning apparatus as defined in claim 3,

wherein the ends of the connections of the separate pass,

ageways that are connected to the positioning means are constructed to be interchangeable, a cam is employed that is adapted to be positioned in one of two reversible positions as a force transmitting connection between the slidable positioning means and a spring-biased roller that is connected to the actuating element, the interchangeability of the connections of the separate passageways with one another and the re-positioning of the cam from one of its positions to the other being operable to change the positioning apparatus from one that provides direct action to one that provides reverse action.

References Cited by the Examiner UNITED STATES PATENTS SAMUEL LEVINE, Primary Examiner. 

1. A FLUID CONTROL DEVICE, COMPRISING TWO PNEUMATIC RELAYS, A WALL PORTION FORMING A SEPARATE CHAMBER FOR EACH RELAY, A PAIR OF SPACED-APART APERTURES FORMED IN THE WALL PORTION OF EACH RELAY TO RESPECTIVELY FORM A FLUID INLET AND EXHAUST PORT FOR EACH OF THE CHAMBERS, A PAIR OF FLEXIBLE MEMBERS POSITIONED IN EACH CHAMBER FORMING OPPOSING WALL PORTIONS THEREOF, A CONNECTING ROD EXTENDING THROUGH AND IN FLUID-TIGHT ENGAGEMENT WITH EACH OF THE FLEXIBLE MEMBERS, A THIRD CHAMBER FORMED BY A STATIONARY WALL PORTION AND ONE OF THE FLEXIBLE MEMBERS, AN INLET PASSAGEWAY FORMED IN THE STATIONARY WALL PORTION OF SAID THIRD CHAMBER FOR APPLYING A FLUID PRESSURE REPRESENTATIVE OF THE MAGNITUDE OF A CONDITION THERETO, A SEPARATE FLAPPER POSITIONED TO SUBSTANTIALLY COVER EACH PORT, A SEPARATE SPRING FOR ONE END OF EACH FLAPPER POSITIONED TO NORMALLY RETAIN EACH FLAPPER IN SUBSTANTIALLY FLUID-TIGHT ENGAGEMENT WITH AN INNER WALL SURFACE OF THE CHAMBER FORMING THE END OF ITS ASSOCIATED PORT, THE OTHER END OF THE FLAPPER COVERING THE EXHAUST PORT IN ONE OF THE RELAYS BEING OPERABLY CONNECTED FOR MOVEMENT WITH THE CONNECTING ROD TO OPEN THE LAST-MENTIONED PORT WHILE THE FLAPPER CONVERING THE INLET PORT IN THE SECOND OF THE FIRST TWO MENTIONED RELAYS IS SIMULTANEOUSLY OPENED UPON THE OCCURRENCE OF AN INCREASE IN THE MAGNITUDE OF THEFLUID IN THE THIRD CHAMBER FROM A PRESELECTED LEVEL, THE OTHER END OF THE FLAPPER CONVERING THE INLET PORT IN SAID ONE OF THE RELAYS BEING OPERABLY CONNECTED FOR MOVEMENT WITH THE CONNECTING ROD TO OPEN THE LAST-MENTIONED PORT WHILE THE FLAPPER CONVERING THE OUTLET PORT IN THE SECOND ONE OF THE AFOREMENTIONED RELAYS IS SIMULTANEOUSLY CLOSED UPON THE OCCURRENCE OF A DECREASE IN THE MAGNITUDE OF THE FLUID IN SAID THIRD CHAMBER FROM THE PRESELECTED LEVEL, A CYLINDER, FIRST AND SCOND INLETS ADJACENT OPPOSITE ENDS OF THE CYLINDER, A PISTON SLIDABLY DIPOSED WITHIN THE CYLINDER, A PASSAGEWAY CONNECTING THE FLUID UNDER PRESSURE IN SAID FIRST CHAMBER BY WAY OF THE FIRST INLET WITH AN INNER PORTION OF THE CYLINDER THAT IS AT ONE END OF THE PISTON, ANOTHER PASSAGEWAY CONNECTING THE FLUID UNDER PRESSURE IN THE SECOND CHAMBER BY WAY OF THE SECOND INLET WITH AN INNER PORTION OF THE CYLINDER THAT IS AT THE OTHER END OF THE PISTON, THE PISTON ROD BEING CONNECTED FOR MOVEMENT WITH THE PISTON, AND A FEEDBACK SPRING AD LINKAGE INTERCONNECTING THE PISTON ROD WITH THE CONNECTING ROD TO APPLY A NEGATIVE FEEDBACK FORCE TO SAID CONNECTING ROD. 